1. Field of the Invention
The present invention relates to an optical modulation device comprising a liquid crystal for a display apparatus, a liquid crystal printer or the like, and particularly to an optical modulation device comprising a liquid crystal having memory properties for imparting good display characteristics to the device.
2. Related Background Art
Ferroelectric liquid crystals (FLC) are positively used for display devices, light valves, etc. from the view point of the advantages thereof with respect to a high speed and good memory properties.
Examples of targets for making use of the above advantages include an optical shutter (valve) array, a high definition display apparatus driven by a simple matrix, a high-density recording light valve combined with a photoconductor and so on. An image display driven by an active matrix comprising thin film transistors (TFT) is also expected.
Further, much effort is put into obtaining a good half tone (gray-scale) which is an indispensable problem in improvement of the display performance of the FLC.
A method of naturally producing irregularities on an electrode substrate or intentionally imparting a micro mosaic pattern, as disclosed in the specification of Japanese Patent Laid-Open No. 59-193427, and a method of obtaining gradation by providing the thickness of an insulating layer with a step-formed distribution, as disclosed in Japanese Patent Laid-Open No. 61-166590, are exemplified as a method of creating a mixed state of white and black (bright and dark) domains in a single pixel. Japanese Patent Laid-Open No. 64-77023 discloses a method of obtaining an alignment having many defects. Many other methods such as a method of providing a pattern of irregularities with a periodic structure have also been proposed.
However, although it was confirmed that the above methods produce a half tone state, there have been demands for a half tone which is further uniformed in pixel or gradation characteristics which can easily be controlled.
Furthermore, it is necessary for maintaining a good contrast to form a liquid crystal alignment in which defects are less observed.
On the other hand, the inventors of the present invention proposed a method of providing gradation by producing a delay of potential transmission in a pixel in Japanese Patent Laid-Open No. 62-119521, a method of providing gradation by forcing a potential gradient to form in a pixel in Japanese Patent Laid-Open No. 62-125330 and others, and a method of providing gradation by forming a gradient in the electrode distance (cell thickness) in Japanese Patent Laid-Open No. 62-145216. All these methods employ a gradient of the field strength applied to a liquid crystal in a pixel for controlling a reverse area of the liquid crystal.
A typical method of the methods disclosed in Japanese Patent Laid-Open Nos. 62-119521 and 62-125330 is a method of forming a basic structure comprising low-resistance electrode lines and a relatively high-resistance film bonded thereto so that a potential gradient is formed in the high-resistance film by delaying transmission of the potential supplied to the electrode lines in the high-resistance film and by supplying different potentials to at least two of the low-resistance electrodes which hold the high-resistance film therebetween in a plane.
The method of providing a potential gradient has the following technical problems to be solved: Since the low-resistance electrode lines generally have considerable resistances, when a potential is applied to the electrode lines from an external power supply, nonuniformity occurs in the voltage applied to the liquid crystal due to a delay of a drop of the potential in the electrode lines between a feed portion and a portion apart from the feed portion. For example, when a matrix panel having about 1000.times.1000 pixels is formed, there is the possibility that local nonuniformity considerably occurs in gradient display.
The method of forming a gradient in the electrode distance which is represented by the method disclosed in Japanese Patent Laid-Open No. 62-145216 sometimes requires a significant difference in the cell thickness for obtaining a large gamma characteristic (typically, saturation voltage.div.threshold voltage) as a gradation characteristic. In this case, the difference in the cell thickness causes a large difference in retardation and thus causes optical coloring, thereby causing difficulties in color display.